1. Field of the Invention
The present invention relates to medical instruments and more particularly to a low noise magnetoencephalogram (MEG) system.
2. Description of the Related Art
The human brain produces both electrical and magnetic signals. It is conventional to detect both ongoing spontaneous electrical brain activity and evoked electrical brain activity (EP) by non-invasive electrodes connected to the scalp of the patient using an electroencephalograph (EEG). For example, evoked brain activity (EP) may be stimulated by an external stimulus such as a flashed light.
The weak magnetic fields at the scalp, produced by the flow of electrical current within the brain, may be detected by a non-invasive magnetoencephalogram (MEG) whose detecting coils are positioned close to the scalp. There are indications, for example, that the magnetoencephalogram may be used to locate the origin of seizures within the human brain. The earliest MEG devices were constructed inside a magnetically shielded room, since the magnetic fields sought to be detected are much weaker than environmental fields. The environmental field, which is "noise" in the system, changes character over time. Even when the magnetic detecting instrument is brought up close to the patient's scalp, the brain's magnetic signals are so faint that they may be drowned out, even after compensating for the earth's magnetic field, by the stronger magnetic field of a truck passing by outside the building. Unfortunately, a properly magnetically shielded room is expensive. The room must be large enough for the patient, the operators and the sensitive equipment and may cost over two hundred thousand dollars.
An alternative to the use of a magnetic shielded room is to improve the signal sensitivity and noise suppression of the magnetic field detector. The preferred type of MEG uses a "SQUID" (superconducting (S) quantum (QU) interference (I) device (D)). The SQUID operates at cryogenic temperature at which metal loses its electrical resistivity. In one type of SQUID a metal film is deposited on a cylindrical quartz cylinder having a narrow bridge "weak link". A magnetic field will increase the energy, making the superconducitng state unstable so that the weak link converts to its normal state (non-superconducting), allowing one flux quantum to enter. The conduction of the ring is monitored by a radio frequency circuit whose sensitivity is increased with a feedback current circuit. Generally a "flux transporter" is used in which a primary coil ("detection coil"), of superconductive wire, is connected to a secondary coil ("input coil") contained in the superconducting chamber with the SQUID. For example, the chamber is a "dewar" vacuum chamber filled with liquid helium. For example, five primary coils are placed closely about the patient's head to detect the brain' s magnetic fluxes. Those fluxes create responses in the detection coils which are communicated to the SQUIDS, which produces a voltage proportional to the net magnetic flux on each detection coil. The flux transporter and the SQUID constitute a "magnetometer". When used to detect brain activity, the system is called a "magnetoencephalograph" or "MEG".
It has been suggested that a MEG system may be used in a magnetically unshielded room by forming the detection coil with oppositely directed loops (a "gradiometer"), so that external magnetic flux will be detected in both loops and their effects will be canceled. The loops may be arranged, for relative insensitivity to spacially uniform fields and gradients, in a "second derivative gradiometer".
However, the brain's spontaneous neuromagnetic activity is weak (10.sup.-12 Tesla) and the neurpmagnetic evoked response is even weaker (10.sup.-13 Tesla). Consequently, it is difficult to detect such activity in the presence of external magnetic fields which produce noise in the system. It has been suggested that the signal/noise ratio may be improved by using an averaging computer which passes signals which are in synchronism with the stimulus. Since the noise is random, the cumulative average should represent the signals as the random noise cancels itself out. Improvements of signal/noise ratio of 10:1 have been reported. However, that technique is applicable only to evoked response (EP) and not to spontaneous brain activity.
Efficient and effective noise cancellation is critical to the development of the magnetoencephalogram (MEG) as a clinical tool. This noise contamination from environmental magnetic fields is a major factor, even in the latest generation, multichannel, second-order gradiometer of DC SQUID (superconducting quantum interference device) systems. For example, for the Biomagnetic Technology Inc. Model 607 magnetometer, removal of this environmental magnetic field contamination requires using the information from four SQUIDS (which act as environmental noise reference sensors) to filter the corresponding noise out of the seven channels which sense both bonafide brain signals and environmental noise. The channels used only to detect environmental noise will be called the noise reference signals or channels, and the channels which detect both neuromagnetic brain signals and environmental noise will be called the brain signals or channels. The noise reference channels monitor the three orthogonal components of the ambient (environmental) magnetic field and, usually, the gradient of the field along the axis of the dewar. In the current commercial systems, these noise reference signals are scaled through a manual procedure which involves static setting of eight independent parameters, four for the noise amplitude and four for the time derivatives of the noise, for each of the seven brain signal channels. This is a total of 56 manual adjustments. Such manual adjustments necessarily are suboptimal and cannot compensate for rapid variations in the relationship between the noise reference and brain signal channels, for example, due to changes in the environment or movement of the dewar from one position to another.
The book, H. Weinberg et al, BIOMAGNETISM: APPLICATIONS AND THEORY (Pergamon Press 1985) contains a chapter, Williamson et al, FIVE CHANNEL SQUID INSTALLATION FOR UNSHIELDED NEUROMAGNETIC MEASUREMENTS (pgs. 46-51, incorporated by reference herein). In Williamson nine SQUIDS are used in an unshielded environment. Five SQUIDS, having second order gradiometers, monitor brain activity (5-channel magnetometer) and four SQUIDS monitor three components of the ambient field and one component of the gradient. The four SQUIDS attempt to cancel background (environmental) noise by an analog noise cancellation system requiring 40 static adjustments for the 5 brain signal channel magnetometer. The results from this cumbersome procedure are that MEG recordings are often unusable because of overwhelming residual contamination by environmental noise sources. In practice, users of MEG devices often must purchase magnetically shielded recording rooms costing about as much as the MEG instrument itself.